Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
304 PM EDT Wed Aug 29 2018

Valid 12Z Sun Sep 02 2018 - 12Z Thu Sep 06 2018

An expansive upper-level ridge which has been persistent across
the north central Pacific for some time is expected to break down
by the start of the extended period, which will allow a somewhat
more progressive flow regime to take hold across Alaska. Models
showed relatively good consensus early in the forecast period with
a shortwave/surface low initially across the eastern Gulf. Farther
north, models are all in general agreement that a compact but
energetic shortwave traversing Arctic mid/upper-level flow will
approach northern Alaska by Sun-Mon. Given the origin of the
system at very high latitudes along with the small scale,
predictability is moderate at best, but model differences are
generally limited to timing. Thus, a majority deterministic model
blend (including the GFS/ECMWF/UKMET) was used as a forecast
starting point during days 4-5.

From day 6 (Tue) onward, the upper flow in the vicinity of Alaska
should become increasingly amplified in response to the upstream
extratropical transition of Typhoon Jebi north of Japan. An
elongated area of shortwave energy trailing the system initially
crossing the Gulf over the weekend looks to undergo amplification
as it moves east across the Gulf. Model solutions differ with
respect to whether the feature will cut off and dig farther south,
or will move more quickly east toward Southeast Alaska and western
Canada. Farther west, an additional shortwave is expected to
quickly amplify as it crosses the Bering Sea and the Aleutians
Tue-Thu. Deterministic and ensemble member solutions show above
average consensus given the time frame with respect to the general
characteristics of this feature and the amplification as it moves
east across the Aleutians, but a range of solutions exist on the
timing. In general a trend toward a slower eastward progression of
this system was noted among the guidance compared to yesterday,
and this is reflected in the WPC forecast. Given increased spread,
a majority ensemble mean (ECENS and GEFS) based blend was
preferred for days 6-8.

The system crossing the eastern Gulf Sun will bring the likelihood
of showers to Southeast Alaska. The North Slope and much of the
Interior will also see the potential for rain/snow showers Sun-Tue
as Arctic shortwave energy traverses the region and a surface
frontal boundary lingers. By early next week, rain chances will
quickly increase across the Aleutians as the next system crosses
the Bering, with precipitation chances increasing for mainland
Alaska by Wed-Thu. Temperatures are expected to be near or
slightly below climatological norms throughout the period.

Ryan


WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can be found at:

http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html